feat: sync Docker images from the controller to remote computes

When a Docker node is created on a remote compute whose Docker daemon
does not have the image, the compute now raises ImageMissingError
instead of blindly pulling from the Docker repository. The controller
exports the image from the Docker daemon on its host (docker save
stream) and streams it to the compute which loads it, so locally built
or docker-loaded images work across computes. When the image is not
available on the controller host either, the compute is asked to pull
it from the Docker repository as a fallback.

- add a POST /docker/images/load compute endpoint that streams a
  docker save tar into the Docker daemon
- let Docker.http_query pass raw (non-dict) request bodies through so
  the tar can be streamed to the daemon
- drop the inline pull from DockerVM.create() and the now unused
  DockerVM.pull_image wrapper

tests/api/routes/compute/test_images.py

@@ -47,3 +47,16 @@ class TestImagesRoutes:
             )
             mock.assert_not_called()
             assert response.status_code == status.HTTP_422_UNPROCESSABLE_CONTENT
+
+    async def test_load_docker_image(self, app: FastAPI, compute_client: AsyncClient) -> None:
+
+        with asyncio_patch("gns3server.compute.docker.Docker.load_image") as mock:
+            response = await compute_client.post(
+                app.url_path_for("compute:load_docker_image"),
+                content=b"docker-save-tar-bytes"
+            )
+            mock.assert_called_once()
+            # the tar is streamed to the Docker daemon as an async iterable
+            stream = mock.call_args[0][0]
+            assert hasattr(stream, "__aiter__")
+            assert response.status_code == status.HTTP_204_NO_CONTENT